Automotive radar systems

ABSTRACT

An automobile radar which serves to provide data for cruise control or other systems in a host vehicle, comprising means for measuring radar boresight misalignment by detecting the presence of apparent variations in the spacing of stationary objects from the direction of motion of the host vehicle and utilising such detection to compensate for any misalignment.

[0001] This invention relates to automotive radar systems and moreespecially but not exclusively it relates to automotive cruise controlsystems which use radar.

[0002] In recent years there has been much interest in the developmentof an adaptive cruise control system for automobiles, more especiallyfor motorway use. Such a system involves measuring the speed of a hostvehicle in which the system is fitted, and the distance of the hostvehicle from a vehicle in front in the same lane. Cruise control thenoperates to control the speed of the host vehicle in order to maintain asafe distance from the car in front and to ensure that a predeterminedmaximum speed is not exceeded. An obvious method of measuring the speedand/or the range of the car in front is to use radar. It is clear thatradar suitable for this purpose must not only have very good angularresolution, since it must be able to discriminate with confidence atlong range between a vehicle in front in the same lane as the hostvehicle, from a vehicle in front in an adjacent lane, but moreover musthave very accurate radar boresight alignment with the direction oftravel of the host vehicle.

[0003] Good angular resolution can be provided using known narrow beamradar techniques but hitherto the necessary boresight alignment accuracyhas been much more difficult, if not impossible to achieve reliably andin a relatively simple manner which lends itself to use in the massproduction of automobiles.

[0004] It is an important object of this invention to provide anautomobile radar system wherein the radar boresight is effectivelyaligned accurately with respect to the direction of travel of a hostvehicle and which is suitable for application to mass production.

[0005] According to the present invention as broadly conceived, anautomobile radar which serves to provide data for cruise control orother systems in a host vehicle, comprises means for measuring radarboresight misalignment by detecting the presence of apparent variationsin the spacing of stationary objects from the direction of motion of thehost vehicle and utilising such detection to compensate for anymisalignment.

[0006] According to one embodiment of the invention, an automobile radarwhich serves to provide data for cruise control or other systems in ahost vehicle, comprises radar apparatus for measuring the bearing ofvehicles and other objects in front of the host vehicle and formeasuring the distance of such vehicles/objects from the host vehicle, aradar Doppler filter for measuring the relative speed ofvehicles/objects in front of the host vehicle, a host vehicle speedsensor, stationary object identifier means which identifies asstationary objects, those objects whose relative speed is equal andopposite to the host vehicle speed as sensed, and computer means which,in dependence upon the change of bearing with range of stationaryobjects thus identified, detects misalignment of the radar boresightwith respect to the direction of motion of the host vehicle and appliesan appropriate correction.

[0007] The system may be arranged to recognise, as radar returns from avehicle ahead in the same lane, only those radar returns which arereceived in a predetermined narrow beam centred on the direction ofmotion of the host vehicle, the aforesaid appropriate correctioncomprising offsetting the beam so that boresight misalignment iscompensated for, whereby the boresight is aligned with the direction ofmotion.

[0008] The system may thus be arranged to take account only of radarreturns from within a predetermined narrow angle centred on thedirection of vehicle motion taking into account any angular boresightmisalignment.

[0009] I will be apparent that when the radar boresight is aligned withthe direction of motion of the host vehicle, the distance to one side ofstationary objects, such as lamp posts and crash barriers, as measuredin dependence upon the rate of change of bearing with range, will remainconstant, and thus the detection of apparent changes in this distancecan be used to detect any boresight misalignment and to correct for itas appropriate.

[0010] Although an automobile radar in accordance with this invention iseminently suitable for use with a cruise control system, it is envisagedthat it may also be suitable for use with other control systems such asautomated emergency breaking systems utilising radar to identify thepresence of objects in the path of a vehicle which might represent acollision hazard.

[0011] A cruise control system which embodies a radar system accordingto the invention may comprise other sensors for measuring or detectingvarious vehicle parameters.

[0012] Such sensors may include, a brake pressure sensor, a gear boxsensor, a steering angle sensor, and a throttle sensor.

[0013] The steering angle sensor may be used in a radar according to theinvention to provide data indicative of a deviation from a normalforward motion direction of the host vehicle thereby to provide dataappertaining to curved directions of host vehicle travel which can betaken into account as appropriate when applying corrections.

[0014] One embodiment of the invention will now be described by way ofexample only with reference to the accompanying drawings in which;

[0015]FIG. 1, is a diagram of a host vehicle in which an automobileradar system is fitted, and;

[0016]FIG. 2, is a block schematic diagram of a cruise control systemembodying the radar system shown in FIG. 1.

[0017] Referring now to FIG. 1, a host car 1, travelling in a direction2, along a motorway lane 3, is fitted with a radar system 4, which isused to detect the presence, range and speed, of vehicles ahead in thesame lane, and which forms a part of a cruise control system. The radarsystem produces a narrow beam along a boresight 5, which isunintentionally offset due to production misalignment, from thedirection of motion of the host car 1, by an azimuth angle φ_(e) so thatradar returns from a stationary object 6, appear to originate from atarget at an azimuth angle of φ_(m). It is clear that if a car 7, infront and in the same lane as the host car 1, is to be reliablydistinguished from vehicles in adjacent lanes, such as the car 8, forexample, the effect of this offset must be compensated for.

[0018] It is apparent that a distance h, by which the stationary object6, is spaced orthogonally from the direction of motion 2, will remainconstant as measured by the radar provided its boresight 5, is alignedwith the direction of motion 2. Thus it is clear that any apparentchange in the distance h, as measured by the radar taking account of thechange in the angle φ_(m), with range, may be used as basis forcalculating a beam correction factor, and a system for doing this willnow be described with reference to FIG. 2.

[0019] Referring now to FIG. 2, a cruise control system 9, receives datafrom a brake pressure sensor 10, a gear box sensor 11, a steering anglesensor 12, a throttle sensor 13, a host vehicle speed sensor 14, and theradar system 4, shown in FIG. 1. The cruise control system itself is notcentral to this invention and will therefore not be described in greatdetail general herein. It conventionally receives data from a pluralityof sensors Transforming to cartesian coordinates (i.e. x=d cos θ_(m),and y=d sin θ_(m))

h=y cos θ_(e) +x sin θ_(e)   iii

[0020] However, for real data there will be some error, e, in themeasurements

∴y cos θ_(e) +x sin θ_(e) −h=e   iv

[0021] The best estimate of h can be found by minimising the sum of thesquares of the errors

[0022] i.e. by minimising ∑e_(i)² = e^(T)e

[0023] where e are the errors written as a vector.

E=e ^(T) e=(x sin θ_(e) +y cos θ_(e) −h1)^(T)(x sin θ_(e) +y cos θ_(e)−h1)   v

[0024] where 1^(T)=(1,1 . . . 1)

[0025] To find the minimum $\begin{matrix}{\frac{\partial E}{\partial\theta_{e}} = {{2\frac{\partial e}{\partial\theta_{e}}e} = 0}} \\{= {{- 2} \times 1^{T}\left( {{x\quad \sin \quad \theta_{e}} + {y\quad \cos \quad \theta_{e}} - {h\quad 1}} \right)}} \\{{h\quad 1^{T}1} = {{1^{T}x\quad \sin \quad \theta_{e}} + {1^{T}y\quad \cos \quad \theta_{e}}}}\end{matrix}$

[0026] but for n measurements: $\begin{matrix}{{{1^{T}1} = {{\sum{1 \times 1}} = n}},{{1^{T}x} = {{\sum{1 \times x_{k}}} = {n\quad \overset{\_}{x}}}},{{1^{T}y} = {{\sum{1 \times y_{k}}} = {n\quad \overset{\_}{y}}}}} \\{\left. \Rightarrow{n\quad h} \right. = {{n\quad \overset{\_}{x}\sin \quad \theta_{e}} + {n\quad \overset{\_}{y}\quad \cos \quad \theta_{e}}}}\end{matrix}$

[0027] Therefore the best estimate of h is:

h={overscore (x)} sin θ_(e) +{overscore (y)} cos θ_(e)

[0028] substituting for h in equation iii

E=x sin θ_(e) +y cos θ_(e) −{overscore (x)} sin θ_(e)1−{overscore (y)}cos θ1

E=(x−{overscore (x)}1)sin θ_(e)+(y−{overscore (y)}1)cos θ_(e)

[0029] Let: x−{overscore (x)}1=x_(i), y−{overscore (y)}1=y₁

E=x _(i) sin θ_(e) +y _(i) cos θ_(e)

[0030] differentiating w. r. t. θ_(e) $\begin{matrix}{\frac{\partial E}{\partial\theta_{e}} = {\left( {{x_{i}^{T}\cos \quad \theta_{e}} - {y_{i}^{T}\sin \quad \theta_{e}}} \right)\left( {{x_{i}\quad \sin \quad \theta_{e}} + {y_{i}\quad \cos \quad \theta_{e}}} \right)}} \\{= {{{x_{i}}^{2}\sin \quad \theta_{e}\cos \quad \theta_{e}} - {{y_{i}}^{2}\sin \quad \theta_{e}\cos \quad \theta_{e}} + {x_{i}^{T}y_{i}\cos^{2}\quad \theta_{e}} - {x_{i}^{T}y_{i}\sin^{2}\theta}}} \\{= 0}\end{matrix}$

[0031] Therefore, the alignment error θ_(e) can be found from theequation below.$\theta_{e} = {\frac{1}{2}{\tan^{- 1}\left( \frac{2x_{i}^{T}y_{i}}{{y_{i}}^{2} - {x_{i}}^{2}} \right)}}$

1. An automobile radar which serves to provide data for cruise controlor other systems in a host vehicle, comprising means for measuring radarboresight misalignment by detecting the presence of apparent variationsin the spacing of stationary objects from the direction of motion of thehost vehicle and utilising such detection to compensate for anymisalignment.
 2. An automobile radar as claimed in claim 1, comprisingradar apparatus for measuring the bearing of vehicles and other objectsin front of the host vehicle and for measuring the distance of suchvehicles/objects from the host vehicle, a radar Doppler filter formeasuring the relative speed of vehicles/objects in front of the hostvehicle, a host vehicle speed sensor, stationary object identifier meanswhich identifies as stationary objects, those objects whose relativespeed is equal and opposite to the host vehicle speed as sensed, andcomputer means which, in dependence upon the change of bearing withrange of stationary objects thus identified, detects misalignment of theradar boresight with respect to the direction of motion of the hostvehicle and applies an appropriate correction.
 3. An automobile radar asclaimed in claim 2, wherein the system is arranged to recognise, asradar returns from a vehicle ahead in the same lane, only those radarreturns which are received in a predetermined narrow beam centred on thedirection of motion of the host vehicle, the aforesaid appropriatecorrection comprising offsetting the beam so that boresight misalignmentis compensated for, whereby the boresight is aligned with the directionof motion.
 4. An automobile radar as claimed in claim 3, wherein thesystem takes account only of radar returns from within a predeterminednarrow angle centred on the direction of vehicle motion taking intoaccount any angular boresight misalignment.
 5. In a cruise controlsystem, an automobile radar as claimed in any preceding claim.
 6. Anautomobile radar as claimed in claim 5, in a cruise control systemcomprising a brake pressure sensor, a gear box sensor, a steerng anglesensor, and a throttle sensor.
 7. An automobile radar as claimed inclaim 6, wherein the steering angle sensor is used to provide dataindicative of a deviation from a normal forward motion direction of thehost vehicle thereby to provide data appertaining to curved directionsof host vehicle travel which are taken into account as appropriate whenapplying corrections.
 8. An automobile radar as substantially ashereinbefore described with reference to the accompanying drawings.
 9. Acruise control system including an automobile radar as claimed in anypreceding claim and as substantially as hereinbefore described withreference to the accompanying drawings.